I think that 140mm fans may become more prevalent, but only as a gimmick. The trend in computer hardware in general is away from high-wattage processors ( = lots of heat output) like the AMD Athlon and Pentium 4 and toward more power-efficient designs. Other heat-producing parts like the Radeon HD2900XT GPU and other high-end video cards vent their heat outside, typically. There really isn't a need for even larger fans than the now-common 120mm fans, unless you just want to run them really slowly to push air very quietly.